Prior art panoramic dental x-ray machines are well known. They include, among others, various structures and mechanisms for orbiting an x-ray source-x-ray film (tubehead-camera) assembly in circular or arcuate paths around the patient's head; for varying film travel speed in accordance with tubehead-camera assembly movements; for shifting the patient in a chair; and for providing continuous or discontinuous type radiographic images. Such structures and mechanisms are disclosed, for example, in U.S. Pat. Nos. 2,798,958; 3,045,118; 3,636,349; and 4,125,774.
Prior art systems require the x-ray film to be sandwiched between conventional intensifying screens, i.e., a double screen and double emulsion film, which screens permit the x-ray film to obtain the image in a shorter period of time and with less x-ray exposure to the patient. These double intensifying screens, with film thereinbetween, move as a unit past a slot disposed centrally in a front panel of the camera. The intensifying screens are large, i.e., about 5".times.12", or 12.70 cm.times.30.48 cm, are expensive and reusable but are easily damaged. For example, cracks, fissures and embedded dirt in the screens provide false images, as do bent screens; electrostatic charges which have developed on screen surfaces create lightning-like patterns on the radiograph and the like.
The present invention employs a single screen and single emulsion film although a double emulsion film may be used. The single screen is significantly smaller in area than the conventional prior art screens, having an active area only about 1/40 in the area thereof. Since the present screen is never flexed, it can be made substantially thicker, resulting in improved detection efficiency.
Intensifying screens are standard items which are commercially available. They fluoresce to emit visible light when struck by x-rays because of substances called phosphors contained within the screen. Each phosphor crystal that absorbs x-ray energy typically emits a bluish-green light whose brightness is related directly to the intensity of the x-rays in that infinitesimally small portion of the image. Thus, over the entire surface of the intensifying screen, differences in x-ray intensity are transformed into differences in light intensity to which the x-ray film is highly sensitive. The entire image is thus "intensified" for recording by the film. Consequently, smaller radiation doses are employed than would be needed without the screens. By increasing the thickness of the screen, as in the present invention, which increases the number of phosphor crystals, detection efficiency increases on the order of about 2 to 1, which increase in efficiency is offset by a loss thereof when only a single screen is used.
A minification lens is mounted after the screen, which lens reduces the panoramic images from the screen while concomitantly increasing the light intensity of the image. The reduced or minified light-intensified images are then recorded on 35 mm roll film, for example. By reducing the film size from the conventional 5".times.12", or 12.70 cm.times.30.48 cm, of 35 mm film size, i.e., about a 14 to 1 reduction in size, which reduction is readily achievable through the practice of the present invention, there will result a savings in silver and film costs along with space required for film and screen storage. The size of the camera may be substantially reduced, not only because of the smaller size film contained therewithin, but the screen will have an input face of only about 51/8".times.0.275", or 13.02 cm.times.0.70 cm, which approximates the dimensions of the camera slot, as opposed to conventional 5".times.12", or 12.70 cm.times.30.48 cm screens. Additionally, the size of the x-ray tubehead and its power supply may be substantially reduced.
The minification lens will typically comprise a fiber optic minification lens, conduit, or plate. The minified, light-intensified images are then recorded on film and subsequently magnified for inspection and/or diagnosis. As later explained, the invention offers up to about 10 to 1 dose reduction and a potential of up to about 7 to 1 image intensification .